1. Field of the Invention
The present invention relates to a machining apparatus and a machining method in which a work is machined by applying voltage to a microscopic gap between a tool electrode and the work to utilize electrical discharge or an electrolytic reaction generated in the gap. More particularly, the present invention relates to an electrical discharge machining method and an electrical discharge machining apparatus that are used for fine machining for forming a microscopic pore, a microscopic hole, a microscopic slit, or the like.
2. Description of the Related Art
Conventionally, the micro electrical discharge machining technology is mainly used as the method of machining a microscopic pore and a microscopic hole, for example, machining a nozzle of an inkjet printer. Recently, an ultra-fine electrical discharge machining apparatus has been developed that can make the hole have a minimum diameter up to 5 μm.
FIG. 20 is a schematic view of an electrical discharge machining apparatus 50 used for making the microscopic hole such as an ink ejection nozzle of an inkjet printer (for example, see Japanese Patent Laid-Open Publication No. H10-202432). The electrical discharge machining apparatus 50 includes a tool electrode 51 and a driving device 55 that vertically moves the tool electrode 51. A work 53 is arranged directly under the tool electrode 51. A leading end of the tool electrode 51 and the work 53 are immersed in the working fluid having dielectric characteristics such as deionized water. When a potential difference and a distance between the tool electrode 51 and the work 53 do not satisfy electrical discharge conditions, the insulating characteristics of the working fluid provides the insulation between the tool electrode 51 and the work 53.
A circuit configuration of the electrical discharge machining apparatus 50 will now be described. A capacitor 58 is connected in parallel between the tool electrode 51 and the work 53, and the capacitor 58 is connected to a rectangular-pulse power supply 61 through a charging resistor 57. The rectangular-pulse power supply 61 includes a reference pulse generator 62 that can arbitrarily set a frequency and a pulse duration. The capacitor 58 is charged in each application of rectangular pulse voltage. At this point, when the driving device 55 brings the tool electrode 51 close to the work 53, the electrical discharge is generated between the tool electrode 51 and the work 53. Because a portion subjected to the electrical discharge in the work 53 is melted and removed, a concave portion having the same shape as the leading end portion of the tool electrode 51 can be formed in the work 53.
Conventionally, oil is used as the working fluid. However, a long machining time is required because the oil has high insulating characteristics and the electrical discharge is difficult to generate. There is a fear that inflammation is caused, and thus, deionized water has been used as the working fluid in recent years. Because water is separated into an H+ ion and an OH− ion through electrolysis, when the voltage is applied between the tool electrode 51 and the work 53 for a long time, ionization of the working fluid progresses to permit conduction between the tool electrode 51 and the work 53. Because the desired electrical discharge cannot be generated during the conduction, there is a problem in that the machining time becomes longer. Further, because a surface of the work is melted by heating the surface through the conduction, there is the problem in that accuracy of machining is made worse. Therefore, in the electrical discharge machining apparatus 50, the ionization of the working fluid (deionized water) is suppressed and the generation of the conduction is suppressed not by continuously applying the voltage, but by using the rectangular-pulse power supply 61 to shorten one voltage applying time.
However, in the configuration of the conventional electrical discharge machining apparatus, there are the following problems. Namely, when the electrical discharge machining is performed in the configuration shown in FIG. 20, a peak current and an electrical discharge time of the capacitor 58 are determined by the capacitance of the capacitor 58 and the resistor 57. When deionized water is used as the working fluid, because voltage is always applied between the tool electrode 51 and the work 53, the working fluid is sometimes ionized to provide the conduction before a predetermined electrical discharge voltage is applied, which results in passage of insufficient electrical discharge current. Further, the work 53 and the tool electrode 51 are reacted with each other to worsen a surface state of the work, and the accuracy of machining is not sufficiently obtained.
Conventionally, in the rectangular-pulse power supply 61, the voltage pulse is generated by turning one switching element on and off, for example as shown in FIG. 5A. However, in the conventional rectangular-pulse power supply 61, reaction speed of the switching element has a limitation. In the case of a power MOSFET, generally a time of hundreds of nanoseconds is required between turning the switching element on and off, and the voltage pulse of tens of nanoseconds cannot be formed. When the voltage pulse has a pulse duration more than hundreds of nanoseconds, as described above, the ionization of the working fluid is generated to provide the conduction, which worsens the surface state of the work. Therefore, it is necessary that ionization concentration is decreased by replenishing the working fluid, or it is necessary that the ionized working fluid is washed away by generating a flow of the working fluid to diffuse the ion and the working fluid is maintained at a non-ionized state between the tool electrode 51 and the work 53. In addition, it is necessary to perform work by temporarily stopping the rectangular-pulse power supply 61 to retract the tool electrode 51 upward. Consequently, as the number of conduction times is increased, the machining time becomes longer.
An object of the invention is therefore to provide an electrical discharge machining method and an electrical discharge machining apparatus in which the accuracy of machining and machining speed are improved.
Another object of the invention is to provide an electrical discharge machining method and an electrical discharge machining apparatus in which the ionization of the working fluid is suppressed to shorten the machining time by adjusting duty ratio of voltage applied to the tool electrode according to a generation state such as the electrical discharge the tool electrode and the work.
A further object of the invention is to provide an electrical discharge machining method and an electrical discharge machining apparatus, in which high accuracy machining of surface roughness can be provided by appropriately setting an applied voltage, a voltage pulse duration, and a number of applied times between the tool electrode and the work.